The basic functionality of a Hall sensor is to measure the magnitude of a magnetic field, based on the so called “Hall-effect”, whereby a voltage is generated over a conductor (e.g. a conductive plate) when a current is flowing through said conductor in the presence of a magnetic field. This phenomenon is well known in the art, and hence need not be further explained here.
There are however several problems related to the readout of a Hall sensor:                1a) the Hall-voltage is typically very small (typically in the microvolt to millivolt range) hence needs to be amplified, but both Hall element and amplifiers may have an offset (the output of the amplifier is non-zero in the presence of a zero magnetic field). This problem is typically addressed in the prior art by measuring the offset-voltage in a calibration (during production), storing the measured value in a non-volatile memory in the device, and retrieving the stored value and subtracting it from the output of the amplifier during actual use of the device;        1b) Another problem is that this offset is not constant over time, but drifts. This problem is addressed in the prior art by using a principle called “spinning current” and/or “chopping”. Switching of polarity combined with current spinning is used to eliminate both offset of the Hall element and offset of the amplifier. Stated in simple terms, this means applying the biasing current (or biasing voltage) not statically to a particular pair of input nodes and reading the result on a particular pair of output nodes, but applying the biasing current (or voltage) consecutively to different nodes of a Hall element (e.g. Hall plate), one at the time, and reading the output over corresponding output nodes, and averaging the results;        2) the Hall voltage is also temperature dependent, inter alia because the above mentioned offset of the Hall element and/or amplifier varies with temperature. This problem is typically addressed in the prior art by measuring the offset value for zero magnetic field at several different temperatures, by storing the measured offset-values in a non-volatile memory in the device, by measuring the temperature of the device during actual use (using a temperature sensor), and by compensating the amplified Hall output value using the stored offset-value for the measured temperature;        3) the Hall voltage is also dependent on mechanical stress (exerted on the Hall element), due to phenomena known as “Piezo-Hall effect” and/or “piezoresistance” effect, and also the output of the temperature sensor mentioned above is dependent on mechanical stress (exerted on the temperature sensor). Such mechanical stress is typically caused by the packaging (e.g. plastic moulded packaging). The physical phenomena of the “piezoresistive effect” and “piezo-Hall effect” are well known, and is highly desirable in stress sensors, but it is undesirable in other devices, such as a Hall sensor device. If mechanical stress would not change over time, this problem could be easily solved by the calibration test, but unfortunately, mechanical stress varies over time, inter alia because of moisture in the packaging.        
Mathematical models of the piezoresistive effect and the piezo-Hall effect are known in the art, wherein mechanical stress is represented as a tensor having 6 independent components. A straight-forward mathematical approach would thus lead to a set of 6 simultaneous equations in 6 stress variables plus one additional variable for the temperature. Such a direct approach is very complex.
U.S. Pat. No. 7,980,138 recognizes the problem of stress dependence and temperature dependence, and proposes (FIG. 5 of said publication) a stress sensor that is relatively independent of temperature. The sensor has a bridge circuit with two branches, each branch has a “n-type resistor L” (i.e. two n-type resistor strips positioned at 90° with respect to each other connected in series) and “a vertical n-type resistor”. Because all resistors are n-type, their temperature behaviour is the same, and thus, their ratio depends primarily on mechanical stress, and only minimal on temperature.
There is still room for improvement or for alternatives.